Plasmonic field-enhanced photodetector and image sensor using light absorbing layer having split conduction band and valence band

ABSTRACT

A plasmonic field-enhanced photodetector is disclosed. The photodetector absorbs surface plasmon polaritons (SPPs) by using a light absorbing layer having a conduction band and a valence band in which an energy is split, the SPPs being generated by combining surface plasmons (SPs) with photons of a light wave, and generates photocurrent based on the absorbed SPPs.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.62/961,029 filed on Jan. 14, 2020, which application is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The disclosure relates to a plasmonic field-enhanced photodetector usinga light absorbing layer having a split conduction band and valence band,and more particularly, to a technique utilizing the phenomenon thatcollective oscillations of free electrons, known as surface plasmons(SPs) are created by coupling between light and free electrons on ametal surface and combined with light waves into surface plasmonpolaritons (SPPs), and the SPPs create a strongly enhanced electricfield at a metal-dielectric interface.

RELATED ART

Similarly to a general metal-oxide-semiconductor field-effect transistor(MOSFET) structure, a photodetector is fabricated in a standardcomplementary metal oxide silicon (CMOS) process, so that current flowsbetween a source and a drain.

However, concentration of incident light on a gate leads to active lightexcitation in polysilicon and doped ions inside the gate in which alight absorbing layer is located, the interface between an oxide filmand a silicon substrate, and the silicon substrate. Further, chargeswhich have absorbed energy tunnel into the gate from a channel regionformed between the oxide film and the silicon substrate or a drainelectrode to which a driving voltage is applied, through the oxide film.This tunneling forms a charge depletion layer between the gate and theoxide film, causing a change in the total charge amount of the gate.

The change in the charge amount and a gate electric field, caused by thetunneling, directly leads to a decrease of the threshold voltage of achannel. After the tunneling, light incident on the gate eventuallyinduces photocurrent in the channel of the transistor. In addition, thegate that receives light is coupled to an external electrode to controloutput signal characteristics such as the intensities of thephotocurrent and dark current of the channel, sensitivity, and a dynamicrange, caused by the light, and to reset pixels.

The photodetector is typically formed to bring about photoelectricconversion by absorbing light into the gate.

The conventional photodetector has limitations in its effectiveness inreducing the size of the detector and the pixel size of an image sensoradopting the photodetector structure because incident light is directlyabsorbed into a gate of a smaller size than the wavelength band of theincident light and thus light excitation does not occur.

Moreover, there is a need for further increasing the internal quantumefficiency of incurring photoelectric conversion from charges directlyexcited by incident light.

SUMMARY

An aspect of the disclosure is to perform photoelectric conversion sothat surface plasmon polaritons (SPPs) directly contribute tophotocurrent, through surface plasmons (SPs) and SPPs created atinterfaces with different dielectric constants by interaction betweenincident light and a metal layer.

Another aspect of the disclosure is to achieve wavelength responsecharacteristics unavailable from pure silicon by creating the effect ofobtaining a plurality of split band gaps through an energy level whichmay be involved in coupling of SPPs.

Another aspect of the disclosure is to increase a signal to noise ratio(SNR) during detection of a long wavelength exceeding the band gapenergy of silicon by much higher quantum efficiency than in the case ofdirect excitation of photons of incident light.

Another aspect of the disclosure is to maximize a light sensingcapability by tunneling charges, which are excited by SPPs generatedthrough interaction between incident light and a metal layer and alocalized electric field effect, into a separate space through aninsulation film, thereby causing a change in the charge amount of thespace and the size of a depletion layer and then affecting the thresholdvoltage of an adjacent current channel.

Additional aspects will be set forth in part in the description whichfollows and, in part, will be apparent from the description, or may belearned by practice of the presented embodiments of the disclosure.

According to an embodiment, a photodetector absorbs surface plasmonpolaritons (SPPs) by using a light absorbing layer having a conductionband and a valence band in which an energy is split, the SPPs beinggenerated by combining surface plasmons (SPs) with photons of a lightwave, and generates photocurrent based on the absorbed SPPs.

According to an embodiment, the light absorbing layer may includesingle-crystal silicon having the conduction band and valence band inwhich the energy level is split by applying uniaxial tensile stress.

According to an embodiment, in the light absorbing layer, the energylevel may be split into a heavy hole and a light hole in the valenceband.

According to an embodiment, the photodetector may generate thephotocurrent by inducing the photocurrent through tunneling caused bythe absorbed SPPs.

According to an embodiment, the photodetector may include a metal layershielding incident light and generating the SPPs by interaction betweenphotons of the shielded incident light and the SPs generated at aninterface with a dielectric, a light absorbing layer absorbing thegenerated SPPs and allowing charges excited by the absorbed SPPs and alocalized electric field effect to tunnel through an insulation film,and a semiconductor layer inducing the photocurrent based on an electricfield effect of tunneled electrons.

According to an embodiment, at least a part of the metal layer mayinclude an opened nanohole, and the generated SPPs may be absorbed intothe light absorbing layer through the nanohole.

According to an embodiment, the nanohole may be formed of a materialhaving a greater dielectric constant than air.

According to an embodiment, the nanohole may be formed of a materialcontaining Si₃N₄, and the metal layer contains at least one of aluminum(Al), gold (Au), or silver (Ag).

According to an embodiment, the metal layer may be extended from asurface to the light absorbing layer and guides the generated SPPs tothe light absorbing layer.

According to an embodiment, an image sensor includes a metal surface, ametal nanohole array formed on the metal surface, and a detector arrayformed at a position corresponding to the metal nanohole array. Thedetector array absorbs SPPs by using a light absorbing layer having aconduction band and a valence band in which an energy level is split,the SPPs being generated by combining SPs formed on the metal surfacewith photons of a light wave, and generates photocurrent by using theabsorbed SPPs.

According to an embodiment, the detector array may generate thephotocurrent by inducing the photocurrent through tunneling caused bythe absorbed SPPs.

According to an embodiment, the metal surface may shield incident lightand generate the SPPs by interaction between photons of the shieldedincident light and the SPs generated at an interface with a dielectric.The detector array may include a light absorbing layer absorbing thegenerated SPPs and allowing charges excited by the absorbed SPPs and alocalized electric field effect to tunnel through an insulation film,and a semiconductor layer inducing the photocurrent based on an electricfield effect of tunneled electrons.

According to an embodiment, the metal surface may be extended from asurface to the light absorbing layer and guide the generated SPPs to thelight absorbing layer.

According to an embodiment, a method of operating a photodetectorincludes absorbing SPPs by using a light absorbing layer having aconduction band and a valence band in which an energy level is split,the SPPs being generated by combining SPs with photons of a light wave,and generating photocurrent by using the absorbed SPPs.

According to an embodiment, the generation of photocurrent may includegenerating the photocurrent by inducing the photocurrent throughtunneling caused by the absorbed SPPs.

According to an embodiment, a method of operating an image sensorincludes generating photocurrent by using a metal nanohole array formedon a metal surface and a detector array formed at a positioncorresponding to the metal nanohole array. The generation ofphotocurrent includes absorbing SPPs by using a light absorbing layerhaving a conduction band and a valence band in which an energy level issplit, the SPPs being generated by combining SPs formed on the metalsurface with photons of a light wave, by the detector array, andgenerating the photocurrent by using the absorbed SPPs by the detectorarray.

According to an embodiment, the generation of the photocurrent mayinclude generating the photocurrent by inducing the photocurrent throughtunneling caused by the absorbed SPPs.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainembodiments of the disclosure will be more apparent from the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a diagram illustrating the structure of a photodetectoraccording to an embodiment of the disclosure;

FIG. 2 is a diagram illustrating an image sensor according to anembodiment of the disclosure;

FIG. 3 is a diagram illustrating a photocurrent generation mechanism inan energy band diagram;

FIG. 4 is a diagram illustrating the structure of a photodetectorincluding strained silicon layers;

FIG. 5 is a diagram illustrating an embodiment of directly generatingsurface plasmon polaritons (SPPs) in a photocurrent channel;

FIG. 6 is a diagram illustrating an embodiment of directly propagatingSPPs to a light absorbing layer based on a metal composition; and

FIG. 7 is a graph illustrating incident photo to current efficiency(IPCE) according to the disclosure.

DETAILED DESCRIPTION

Specific structural or functional descriptions of embodiments accordingto the concept of the disclosure are mere examples presented only forthe purpose of illustrating the embodiments according to the concept ofthe disclosure. The embodiments according to the concept of thedisclosure can be implemented in various forms, not limited to theembodiments described herein.

It should be appreciated that various embodiments of the presentdisclosure and the terms used therein are not intended to limit thetechnological features set forth herein to particular embodiments andinclude various changes, equivalents, or replacements for acorresponding embodiment.

The term as used in the disclosure, “1^(st)”, “2^(nd)”, “first” “second’may be used to describe various components, not limiting the components.These expressions are used to distinguish one component from anothercomponent. For example, a first component may be referred to as a secondcomponent and vice versa without departing the scope of the disclosure.

When it is said that a component is “coupled with/to” or “connected to”another component, it should be understood that the one component isconnected to the other component directly or through any othercomponent. On the other hand, when it is said that a component is“directly connected to” or “directly coupled to” another component, itmay be understood that there is no other component between thecomponents. Expressions describing the relationship between components,for example, “between” and “immediately between” or “directlyneighboring to” should be interpreted similarly.

The terms as used herein are intended only to describe specificembodiments, not to limit the disclosure. It is to be understood that asingular form of a noun corresponding to an item may include one or moreof the things, unless the relevant context clearly indicates otherwise.In the disclosure, the term “have”, “may have”, “include”, or “mayinclude” signifies the presence of a corresponding feature, number,function, operation, component, or part, or a combination of thereof,not excluding the presence of one or more other features, numbers,functions, operations, components, or parts, or combinations of thereof.

Unless otherwise defined, the terms including technical or scientificterms used in the disclosure may have the same meanings as generallyunderstood by those skilled in the art. The terms as generally definedin dictionaries may be interpreted as having the same or similarmeanings as or to contextual meanings of related technology. Unlessotherwise defined, the terms should not be interpreted as ideally orexcessively formal meanings.

Embodiments will be described below in detail with reference to theattached drawings. However, the scope of the disclosure is not limitedto these embodiments. Through the drawings, like reference numeralsdenote the same components.

FIG. 1 is a diagram illustrating the structure of a photodetector 100according to an embodiment of the disclosure.

The photodetector 100 according to an embodiment may perform theoperations of absorbing surface plasmon polaritons (SPPs) generated bycombining surface plasmons (SPs) with photons of a light wave by using alight absorbing layer having a conduction band and a valence band inwhich an energy level is split, and generating photocurrent. Thephotodetector 100 may generate the photocurrent by inducing thephotocurrent through tunneling caused by the absorbed SPPs.

In the disclosure, photoelectric conversion may be performed such thatSPPs directly contribute to photocurrent through SPs and SPPs generatedat an interface with different dielectric constants by interactionbetween incident light and a metal layer formed to shield the incidentlight for the purpose of maximizing photoelectric conversion quantumefficiency for the incident light. Further, in the disclosure, it ispossible to perform photoelectric conversion that maximizes a lightsensing capability by tunneling charges, which have been excited by theSPPs generated through the interaction between the incident light andthe metal layer and through a localized electric field effect, into aseparate space through an insulation film, and thus changing the chargeamount of the space and the size of a depletion layer, thereby affectingthe threshold voltage of a nearby current channel and maximizing thelight sensing capability. Herein, an energy level may be split byinserting a strained silicon layer in a space available for coupling ofthe SPPs and thus controlling localized level generation and the size ofthe light absorbing layer. Further, light absorption responsecharacteristics may be extended to a wavelength band of 1200 nm or morewhich a conventional silicon photodetector cannot sense, by the splitenergy levels.

Further, according to an embodiment, the photodetector 100 may generatethe photocurrent by inducing the photocurrent through tunneling causedby the absorbed SPPs.

Specifically, the photodetector 100 according to an embodiment mayinclude a metal layer 102, a light absorbing layer 104, andsemiconductor layers 105, 106, 107, and 108.

The metal layer 102 shields incident light 101, and photons of theshielded incident light 101 interacts with SPs generated at an interfacewith a dielectric, thereby generating the SPPs.

Further, the light absorbing layer 104 absorbs the generated SPPs andallows charges excited by the absorbed SPPs and a localized electricfield effect to tunnel through the semiconductor layers 105, 106, 107,and 108 via an insulation film.

The semiconductor layers 105, 106, 107, and 108 may induce thephotocurrent based on an electric field of tunneled electrons.

For this purpose, Node A 106, Node B 107, and Node D 108 may be formedwith different impurity doping polarities and concentrations on asilicon substrate, as illustrated in FIG. 1. Further, Node C 104 may beformed of polysilicon or a metal, with the insulator 105 interposedbetween Node C 104 and the nodes 106, 107, and 108.

According to an embodiment, the photodetector 100 may be formed byshielding a whole light incident surface with the metal layer 102, andartificially forming a metal layer-free nanohole 103 of a dielectricmaterial with a large dielectric constant, not a metal, over Node C asthe light absorbing layer 104.

According to an embodiment, the nanohole 103 may be formed of a materialwith a greater dielectric constant than air. For example, the nanohole103 may be formed of a material containing Si₃N₄.

In this structure, when visible light and near-infrared light reach thesurface, the wavelengths of the light are larger than the size of thenanohole 103. Therefore, the light fails to directly pass through themetal-free part and thus generate semiconductor excitons.

Instead, the incident light is coupled with the metal into SPPs. TheSPPs propagate along the interface between the metal layer 102 and thedielectric on the surface and then meet the nanohole 103.

The SPPs form localized surface plasmons (LSPs) at edges where the metallayer 102 meets the dielectric. The SPPs cause strong electric fieldenhancement, thereby generating an electric field toward the siliconsubstrate.

Further, the SPPs are increased in momentum and energy based on a wavevector changed by the incident light and effectively react with excitonsof polysilicon, a silicon oxide film, and the substrate, therebyexciting lots of charges.

The excited charges tunnel into Node C 104 through the silicon oxidefilm by the strongly enhanced electric field at edges of the nanohole103 on the metal surface, and thus changes the total charge amount ofNode C 104.

The change of the charge amount of Node C 104 in turn changes thethreshold voltage of a current channel formed at the interface betweenthe silicon oxide film and the silicon, resulting in induction ofphotocurrent.

When a large number of electrons are introduced into Node C 104 or NodeC 104 is formed of a metal, the SPPs converted by the incident light maybe generated between Node A 106 and Node B 107 and thus make a directcontribution as photocurrent.

In the disclosure, the photodetector utilizes the phenomenon that in theworld of nano photonics, the light speed is not constant, light may bestopped, and a strongly enhanced electric field is generated at ametal-dielectric interface by SPPs being a combination of light and SPswhich are collective oscillations of free electronics through couplingbetween the light and the free electrons on the metal surface.

This phenomenon is that ordinary light interacts with a metal and isconverted to SPPs. The wavelength of the light is compressed to reactwith a semiconductor, with the new physical properties of increasedmomentum and energy. Further, as this phenomenon changes the size andstructure of the surface metal layer, the resonance spectrum or thedispersion peak of the SPPs may be controlled. Then, a very efficientand innovative photoelectric conversion characteristic may be achievedby some special quantum mechanical designs in the operation andstructure of the photodetector.

The photodetection characteristics of a photodetector basically adoptinga PN junction structure or an existing photo assistedtunneling-photodetector (PAT-PD) relies on the principle of convertingcharges excited directly from the photon energy of incident light into acurrent or voltage. Therefore, the internal quantum efficiency ofphotoelectric conversion may be greatly affected by the size of thedetector or the angle of incident light. In the disclosure, however, thecharacteristics may be extended to an image sensor by applying astructure and operation principle that combine SPPs resulting frominteraction between light and a metal with the quantum tunnelingphenomenon to a novel photodetector.

That is, when SPPs generated by irradiating light onto SPs existing at ametal-dielectric interface meet a nano-sized artificial metal structure,localized plasmonic field enhancement occurs, and the wave vector of theSPPs becomes larger than that of the incident light. When it is saidthat the wave vector increases, this means that the energy and momentumbecome greater and the wavelength becomes smaller than those of theincident light. Photoelectric characteristics with a very high internalquantum efficiency may be achieved by converting incident light intoSPPs on a metal surface and coupling the SPPs with a semiconductor, foruse in light excitation, rather than by generating excitons, that is,electron-hole pairs through direct absorption of light into thesemiconductor. This implies that a material with a large dielectricconstant should be selected and a photodetector should be speciallyfabricated in terms of structure, in order to efficiently generate andutilize SPPs.

The operation principle and structure of photoelectric conversionaccording to an embodiment are described below. A detector of a PNjunction structure with a low quantum efficiency is not used as a lightreceiving portion. Particularly, the light receiving portion isfabricated in a structure in which SPPs generated on a metal surface bylight interact with excitons generated from a semiconductor, theresulting excited electrons are fast tunneled into a specific spacethrough an insulator, and then photocurrent is induced by the electricfield effect of the tunneled electrons. The electric field that causesefficient tunneling mainly relies on LSPs generated by the SPPS andnano-structures, and a plasmonic field enhancement effect. Morespecifically, the surface metal layer of the light receiving portion isopened to a size of couple of tens nano-meters to couple of hundredsnano-meters, and the remaining part of the surface metal layer isshielded, to partially generate electric field enhancement. A materialsuch as Si3N4, which has a greater dielectric constant than air, is usedfor the metal-free open part, and the thickness of the material iscontrolled so that incident light may reach the interface between thedielectric and the metal. Then, photons interact with SPs at themetal-dielectric interface, thereby generating SPPs. When the SPPs meetthe nano-sized non-metallic dielectric during propagation on the metalsurface, LSPs are generated and the electric field enhancementphenomenon occurs to edges at which the metal ends. The electric fieldenhancement tunnels charges excited by the SPPs, and efficiently inducesphotocurrent.

However, even though the incident light is converted into SPPs withlarge energy and momentum and interacts with excitons, excitation ofexcitons which are in a long wavelength band and have smaller energythan the band gap energy of silicon, for example, in a wavelength equalto or longer than 1200 nm should be processed separately as follows.

First, artificial localized interface states are generated at a specificpart of a light absorption path within the band gap of silicon. Ingeneral, before device fabrication, the bonding state of the danglingbond of initial silicon is controlled by some surface cleaning of asilicon substrate, to facilitate formation of excitons bylong-wavelength absorption. However, localized states may be generatedseparately by combining one to two mono layers of a material such as Nor Ge with a silicon surface and then forming an oxide layer. Thelocalized quantum states generated within the band gap transitionelectron-hole pairs and convert the electron-hole pairs to an electricsignal, through interaction with the SPPs.

In this case, the density of the localized quantum states is generallynot high, which makes it difficult to expect efficient photoelectricconversion characteristics. In the disclosure, however, photoelectricconversion utilizes the phenomenon that SPPs are generated by bringingincident light in a long wavelength equal to or longer than 1200 nm intocontact first with a metal, and thus the wave vector of the SPPs becomelarger than that of the incident light, without generating excitonsdirectly from the incident light.

That is, it is possible to convert incident light into SPPs with thesame frequency and a changed light speed, propagate the SPPs to anano-sized light receiving portion, and very efficiently exciteelectrons trapped in localized quantum states.

In reality, since the area of a light absorbing layer (light receivingportion) directly affects light reception efficiency in a typicalphotodetector being a PN junction diode, an area in which light isincident should be increased to achieve high sensitivity. However, asthe light absorption area decreases, the photoelectric conversionefficiency increases, in view of the structure and operation principlein which charges tunneled by the plasmonic field enhancement effect neara nanohole on a nano-sized metal thin film change the charge amount of agate and thus photocurrent is induced, in the disclosure.

This is because the wave vector of the SPPs generated from interactionbetween incident light and a metal becomes larger than the wave vectorof the incident light, and thus energy and momentum for excitingexcitons increase. That is, as the size of the metal hole decreases, thedensity of a localized electric field per unit area increases. Hence,the probability of tunneling excited charges through an oxide filmincreases and strong photocurrent is generated accordingly, therebyincreasing photoelectric conversion efficiency. Although aluminum isusually available for the metal layer, a precious metal such as gold(Au) or silver (Ag) may be additionally coated on the surface of themetal layer to further activate the generation of SPPs.

FIG. 2 is a diagram illustrating an image sensor 200 according to anembodiment of the disclosure.

The image sensor 200 according to an embodiment may include a metalsurface 202, a metal nanohole array 201 formed on the metal surface 202,and a detector array 203 formed at a position corresponding to the metalnanohole array 201.

A method of operating the image sensor 200 according to an embodimentmay include generating photocurrent by using the metal nanohole arrayformed on the metal surface and the detector array formed at theposition corresponding to the metal nanohole array.

In the method of operating the image sensor 200, the generation ofphotocurrent may include absorbing SPPs generated by combining SPsformed on the metal surface with photons of a light wave, and generatingthe photocurrent by using the absorbed SPPs, in the detector array.

Particularly, in the method of operating the image sensor 200 accordingto an embodiment, the SPPs may be absorbed by a light absorbing layerhaving a conduction band and a valence band in which an energy level issplit. For this purpose, the light absorbing layer of the image sensor200 may include single-crystal silicon having a conduction band in whichan energy level is split by applying uniaxial tensile stress. Further,the energy level may be split into a heavy hole and a light hole in thelight absorbing layer.

According to an embodiment, the metal surface 202 may shield incidentlight, and the photons of the shielded incident light may interact withSPs generated at an interface with a dielectric, thereby generating theSPPs.

Further, the detector array 203 according to an embodiment may include alight absorbing layer 206 and semiconductor layers 207, 208, 209, and210.

The light absorbing layer 206 represented as Node C may absorb thegenerated SPPs, and tunnel charges excited by the absorbed SPPs and thelocalized electric field effect through an insulating film.

Further, the semiconductor layers 207, 208, 209, and 210 may inducephotocurrent based on the electric field of tunneled electrons.

For example, the detector array 203 may absorb SPPs generated bycombining photons of a light wave with SPs formed on the metal surface202, to generate photocurrent.

According to an embodiment, the metal nanohole array 201 may be formeddirectly on the detector array 203. The distance between metal nanoholesis at least a few micrometers, which makes it substantially difficult toallow the SPPs to propagate from a metal nanohole to its adjacent metalnanohole and affect the adjacent metal nanohole.

As illustrated in FIG. 2, the momentum and energy of the SPPs increasebased on a wave vector converted by incident light 204. Further, theSPPs with the increased momentum and energy may excite lots of chargesby more efficiently reacting with excitons in polysilicon, a siliconoxide film, and a substrate.

The excited charges may tunnel into Node C 206 corresponding to thelight absorbing layer through the silicon oxide film by a stronglyenhanced electric field at edges of a metal nanohole 201 on the metalsurface 202, thus changing the total charge amount of Node C 206.Further, a depletion region is generated in a part adjacent to the oxidefilm due to the changed total charge amount, affecting the electricfield of the channel. As a result, the change in the charge amount ofNode C 206 leads to a change in the threshold voltage of the currentchannel formed at the interface between the silicon oxide film andsilicon and hence induction of photocurrent. Further, when the voltageof Node C 206 is externally adjusted, output characteristics such assensitivity, a dynamic range, and signal linearity may be freelyadjusted.

FIG. 3 is a diagram illustrating a photocurrent generation mechanism inan energy band diagram.

FIG. 3 illustrates an operation principle based on an energy level alonga cross-section of a photodetector 300 in a direction 301 and anoperation principle based on an energy level along a cross-section ofthe photodetector 300 in a direction 302.

Regarding the direction 310, SPPs may interact with the energy level ofeach of a poly-silicon gate, a strained silicon layer, and bulk silicon,thereby generating electron-hole pairs. Further, electrons excitedsimultaneously with the generation of the electron-hole pairs tunnelthrough a channel formed at a silicon interface by an electric field andthe oxide film in Node B, as indicated by reference numeral 310.Further, the electrons deplete the area of Node C between P-doped Node Cand the oxide film along with the tunneling.

Regarding the direction 320, Node C to which depletion has occurred dueto the tunneling of the electrons experiences a change in its totalcharge amount. This substantially affects a threshold voltage betweenNode A and Node B. This amounts to the effect of changing the size of anelectric field between current channels affected by the change of thecharge amount in Node C. Therefore, photocurrent is generated throughthe channel according to a variation in channel conductance.

A final factor affecting the channel conductance is the tunneling ofelectrons excited by the SPPs. That is, interaction with the SPPs allowseach energy level to determine the wavelength response characteristicsof the photodetector. If light incoming is stopped, the photodetector isplaced in a dark state, and thus SPPs are neither generated norinteract, the electrons that has tunneled into and depleted Node Ctunnel again to Node C and the channel region. Therefore, the chargeamount of Node C returns to its original value. Further, the depletionlayer tends to decrease, and the photocurrent caused by the SPPsnaturally disappears in the channel.

FIG. 4 is a diagram illustrating the structure of a photodetector 400including strained silicon layers 402 and 403.

Single-crystal silicon has crystal directions [100], [010] and [001]according to orientations of lattices. In the energy band of thesingle-crystal silicon, the lowest level of a conduction band is notequal to the highest level of a valence band in the k-space. Due to thisproperty, electrons excited by light make indirect transition with theassistance of phonons. Therefore, the energy levels of a heavy hole anda light hole are equal at the highest energy level of the valence band,and thus a band gap energy may be simply defined.

However, when uniaxial tensile stress is applied to the single-crystalsilicon, the energy level is split into the heavy hole and the lighthole in the valence band.

Herein, the energy level is split based on the crystal directions [100]and [001] and the crystal direction [010] in the conduction band and thevalence band. Eventually, the effect of splitting the energy level thatmay be involved in coupling of SPPs to obtain a plurality of band gapsis achieved, thereby resulting in wavelength response characteristicsunavailable from pure silicon.

This behavior of the SPPs leads to a much higher quantum efficiency thanin the case of direct excitation of photons of incident light.Therefore, an SNR may be increased during detection of a long wavelengthexceeding the band gap energy of silicon.

For this purpose, the photodetector 400 according to an embodimentincludes a light absorbing layer including a strained silicon layer.Further, the photodetector 400 may absorb incident light in a wavelengthof 1200 nm or more by generating localized levels and splitting anenergy level, using the strained silicon layer.

Specifically, the photodetector 400 according to an embodiment mayabsorb SPPs generated by combining SPs with photons of a light wave,using the light absorbing layer having the energy-split conduction bandand valence band.

The light absorbing layer represented as Node C has the energylevel-split conduction band and valence band by applying uniaxialtensile stress. Further, the energy level may be split into a heavy holeand a light hole in the light absorbing layer.

FIG. 5 is a diagram illustrating an embodiment 500 of directlygenerating SPPs in a photocurrent channel.

When SPPs generated by incident light 501 propagates on the surface of ametal layer 502 and encounters a dielectric with a different dielectricconstant from that of a metal nanohole, the wave vector of the SPPschanges. The newly generated SPPs 503 propagate toward a semiconductorsubstrate 504. Since a resonance frequency changes according to the sizeof the nanohole, the size of the nanohole may be adjusted according to awavelength of incident light to be detected.

Particularly, when the newly generated SPPs are induced to a channelbetween a silicon substrate and an oxide film 505, a bias is applied toan electrode between Node A and Node B, so the SPPs flow directly asphotocurrent through Node B.

Further, lots of electrons are tunneled to Node C to have propertiessimilar to those of a metal. These electrons may create a situation inwhich the SPPs directly contribute to photocurrent as well as lead todirect induction of the SPPs to the interface of the dielectric of theoxide film 505 contacting Node C and interaction between the SPPs andexcitons.

FIG. 6 is a diagram illustrating an embodiment 600 of directlypropagating SPPs to a light absorbing layer according to a metalcomposition.

According to an embodiment, a metal surface 601 may extend from asurface to a light absorbing layer 602 to guide generated SPPs to thelight absorbing layer 602.

A detector array may generate photocurrent by absorbing the SPPsgenerated by combining photons of a light wave with SPs formed on themetal surface 601.

The embodiment 600 is a structure of extending the metal surface 601 ina semiconductor depth direction to guide the SPPs, to maximizephotoelectric conversion based on the SPPs. When the SPPs come intodirect contact with Node C, the SPPs may directly bring electric fieldenhancement to a channel in addition to the LSP effect of a nanohole.Therefore, photocurrent may be induced efficiently without relying onthreshold voltage modulation based on tunneling of charges. Further, theprobability of interacting the SPPs with an oxide film and a strainedlayer on the interface of the oxide film increases, and thus extendedlong wavelength response characteristics may be achieved efficiently.

FIG. 7 is a graph 700 illustrating incident photon to current efficiency(IPCE) according to the disclosure.

In the graph 700, the horizontal axis represents wavelength (nm), andthe vertical axis represents efficiency.

The graph 700 may reveal photoelectric conversion efficiency based onthe enhanced electric field and tunneling effect caused by SPPs based oninteraction between incident light and a metal layer. Specifically, thegraph 700 illustrates IPCE measurements obtained by changing thewavelength band of incident light in a detector structure having a200-nm nanohole metal layer. The measurements on the graph 700 arepercentages of the number of charges in photoelectrically convertedoutput current with respect to the number of photons of incident lightas an input.

As noted from the graph 700, a very high photoelectric conversionefficiency (6000 to 16000%) is observed in a 450 to 950 nm region inwhich photons of incident light are directly absorbed into asemiconductor and thus does not generate excitons.

Particularly, the graph 700 shows that as the wavelength of incidentlight increases, the efficiency increases, when the resonance conditionof the enhanced electric field of LSPs is near the wavelength of 800 nmaccording to a nanohole size.

As is apparent from the foregoing description, according to anembodiment, photoelectric conversion may be performed such that SPPs maydirectly contribute to photocurrent through SPs and SPPs which aregenerated at an interface of different dielectric constants byinteracting a metal layer with incident light.

According to an embodiment, wavelength response characteristicsunavailable from pure silicon may be achieved by creating the effect ofa plurality of split band gaps through an energy level involved incoupling of SPPs.

According to an embodiment, an SNR may be increased during detection ofa long wavelength exceeding the band gap energy of silicon by muchhigher quantum efficiency than in the case of direct excitation ofphotons of incident light.

According to an embodiment, charges excited by the SPPs generatedthrough interaction between the incident light and the metal layertunnel into a separate space through an insulation film, changing thecharge amount in the space and the size of a depletion layer and thenaffecting the threshold voltage of an adjacent current channel.Therefore, a light sensing capability may be maximized.

While the embodiments have been described with reference to the limiteddrawings, those skilled in the art could make many variations andmodifications from the above description. For example, even though thedescribed technique is implemented in a different order from thedescribed method and/or components such as the described system,structure, device, circuit, and so on are combined in a different manneror replaced with other components or their equivalents, appropriateresults may be achieved.

Accordingly, other implementations, other embodiments, and equivalentsto the appended claims fall within the scope of the appended claims.

What is claimed is:
 1. A photodetector comprising: a metal layer thatshields incident light and generates surface plasmon polaritons (SPPs);a light absorbing layer that absorbs the generated SPPs and allowscharges excited by the absorbed SPPs and a localized electric fieldeffect to tunnel through an insulation film; a dielectric includingnanoholes in which at least a part of the metal layer is opened andformed over the light absorbing layer; and a semiconductor layerinducing a photocurrent based on an electric field effect of tunneledelectrons, wherein the SPPs are generated by interaction between photonsof the shielded incident light and surface plasmons (SPs) generated atan interface between the metal layer and the dielectric, wherein theSPPs are absorbed into the semiconductor layer by using the lightabsorbing layer having a conduction band and a valence band in which anenergy is split, the SPPs being generated by combining the SPs withphotons of a light wave, and generating the photocurrent based on theabsorbed SPPs, and wherein the SPPs form localized surface plasmons(LSPs) at edges where the metal layer meets the dielectric, and densityof a localized electric field increases per unit area to increase thephotocurrent as a size of the nanoholes decreases.
 2. The photodetectoraccording to claim 1, wherein the light absorbing layer includessingle-crystal silicon having the conduction band and valence band inwhich the energy level is split by applying uniaxial tensile stress. 3.The photodetector according to claim 1, wherein in the light absorbinglayer, the energy level is split into a heavy hole and a light hole inthe valence band.
 4. The photodetector according to claim 1, wherein thephotocurrent is generated by inducing the photocurrent through tunnelingcaused by the absorbed SPPs.
 5. The photodetector according to claim 1,wherein at least a part of the metal layer includes an opened nanohole,and the generated SPPs are absorbed into the light absorbing layerthrough the nanohole.
 6. The photodetector according to claim 5, whereinthe nanohole is formed of a material having a greater dielectricconstant than air.
 7. The photodetector according to claim 5, whereinthe nanohole is formed of a material containing Si₃N₄, and the metallayer contains at least one of aluminum (Al), gold (Au), or silver (Ag).8. The photodetector according to claim 1, wherein the metal layer isextended from a surface to the light absorbing layer and guides thegenerated SPPs to the light absorbing layer.
 9. An image sensorcomprising: a metal surface; a metal nanohole array formed on the metalsurface; and a detector array formed at a position corresponding to themetal nanohole array, wherein the metal surface is configured to shieldincident light and generate surface plasmon polaritons (SPPs), andwherein the detector array comprises: a light absorbing layer absorbingthe generated SPPs and allowing charges excited by the absorbed SPPs anda localized electric field effect to tunnel through an insulation film;the dielectric including nanoholes in which at least a part of the metalsurface is opened and formed over the light absorbing layer; and asemiconductor layer inducing a photocurrent based on an electric fieldeffect of tunneled electrons, wherein the SPPs are generated byinteraction between photons of the shielded incident light and surfaceplasmons (SPs) generated at an interface between the metal surface andthe dielectric, wherein the detector array absorbs the SPPs into thesemiconductor layer by using the light absorbing layer having aconduction band and a valence band in which an energy level is split,and wherein the SPPs form localized surface plasmons (LSPs) at edgeswhere the metal layer meets the dielectric, and density of a localizedelectric field increases per unit area to increase the photocurrent as asize of the nanoholes decreases.
 10. The image sensor according to claim9, wherein the detector array generates the photocurrent by inducing thephotocurrent through tunneling caused by the absorbed SPPs.
 11. Theimage sensor according to claim 9, wherein the metal surface is extendedfrom a surface to the light absorbing layer and guides the generatedSPPs to the light absorbing layer.
 12. A method of operating aphotodetector, the method comprising: absorbing, at a light absorbinglayer, surface plasmon polaritons (SPPs) by using the light absorbinglayer having a conduction band and a valence band in which an energylevel is split, the SPPs being generated by combining surface plasmons(SPs) with photons of a light wave; and generating, at a semiconductorlayer, photocurrent by using the absorbed SPPs, wherein the SPPs formlocalized surface plasmons (LSPs) at edges where a metal layer meets adielectric including nanoholes in which at least a part of the metallayer is opened, and density of a localized electric field increases perunit area to increase the photocurrent as a size of the nanoholesdecreases, and wherein the dielectric is formed over the light absorbinglayer.
 13. The method according to claim 12, wherein the generation ofphotocurrent comprises generating the photocurrent by inducing thephotocurrent through tunneling caused by the absorbed SPPs.
 14. A methodof operating an image sensor, the method comprising: generatingphotocurrent by using a metal nanohole array formed on a metal surfaceand a detector array formed at a position corresponding to the metalnanohole array, wherein the generation of photocurrent comprises:absorbing, at a light absorbing layer, surface plasmon polaritons (SPPs)by using the light absorbing layer having a conduction band and avalence band in which an energy level is split, the SPPs being generatedby combining surface plasmons (SPs) formed on the metal surface withphotons of a light wave, by the detector array; and generating, at asemiconductor layer, the photocurrent by using the absorbed SPPs by thedetector array, wherein the SPPs form localized surface plasmons (LSPs)at edges where the metal surface meets a dielectric including nanoholesin which at least a part of the metal surface is opened, and density ofa localized electric field increases per unit area to increase thephotocurrent as a size of the nanoholes decreases, and wherein thedielectric is formed over the light absorbing layer.
 15. The methodaccording to claim 14, wherein the generation of the photocurrentcomprises generating the photocurrent by inducing the photocurrentthrough tunneling caused by the absorbed SPPs.